*Yukimara Sanada must be be Toyotomi's general.
*You must play as Tokugawa
*You must keep Toyotomi from invading other fiefs
*You must conquer all other fiefs (not sure if ally counts, I got all allies through threaten)
*Must be done by year 1615 (possibly 1614, either way you should have plenty of time)

Just make sure you completely surround Toyotomi and don't let him take over any more fiefs than the 2 he starts out with, it should be very simple

you know whats more simple. playing AS toyotomi, and just defending tokugawa. worked out perfect for me and i never had to attack anything just defend. and by playing as toyotomi i just stayed as 2 fiefs!